1. Field of the Invention
The present invention relates to a thin film magnetic sensor and a method of manufacturing the same, and particularly to a thin film magnetic sensor suitable for detection of information on the rotation of an axle of a car, a rotary encoder, and an industrial gear, etc., suitable for detection of information on the stroke position of a hydraulic cylinder or an air type cylinder, and on the position and speed of a slide of a machine tool, suitable for detection of information on the arc current of the industrial welding robot, and suitable for use in a geomagnetism direction sensor, and to a method of manufacturing the particular thin film magnetic sensor.
2. Description of the Related Art
A magnetic sensor is an electronic device for converting the detected value of an electromagnetic force such as current, voltage, an electric power, a magnetic field, or a magnetic flux, the detected value of a dynamic quantity such as a position, speed, acceleration, displacement, distance, tension, pressure, torque, temperature, or humidity, and the detected value of the biochemical quantity, into voltage via a magnetic field. A magnetic sensor is classified into, for example, a hole sensor, an anisotropic magnetoresistance (AMR) sensor, and a giant magnetoresistance (GMR) sensor in accordance with the detecting method of a magnetic field.
Among the magnetic sensors noted above, the GMR sensor is advantageous in that:
(1) The GMR sensor has the maximum value in the rate of change of the electrical resistivity, i.e., MR ratio given below, which is markedly larger than that of any of the hole sensor and the AMR sensor:
MR ratio=Δρ/ρo, where Δρ=ρH−ρo, in which ρH denotes the electrical resistivity under the external magnetic field H, and ρo denotes the electrical resistivity under the condition that the external magnetic field is zero.
(2) The GMR sensor has a change with temperature in the resistance value, which is smaller than that of the hole sensor.
(3) Since the material producing the giant magnetoresistance effect is a thin film material the GMR sensor is suitable for miniaturization of the magnetic sensor.
Such being the situation, it is expected for the GMR sensor to be used as a high sensitivity micro magnetic sensor in a computer, a power generator, a car, a household electrical appliance, and a portable device.
The materials that are known to exhibit the GMR effect include, for example, (1) a metallic artificial lattice, which is a multilayer film including a ferromagnetic layer such as a layer of Permalloy and a nonmagnetic layer such as a layer of Cu, Ag, or Au, or a multilayer film having a four layer structure, which called a spin valve, the spin valve including an antiferromagnetic layer, a ferromagnetic layer (pinned layer), a nonmagnetic layer, and a ferromagnetic layer (free layer), (2) a metal-metal system nano granular material provided with particulates of a nanometer size formed of a ferromagnetic metal such as Permalloy and with a intergranule consisting of a nonmagnetic metal such as Cu, Ag, or Au, (3) a tunnel junction film that is allowed to exhibit the MR effect by the spin-dependent tunneling effect, and (4) a metal-insulator system nano granular material provided with particulates of a nanometer size formed of an alloy of a ferromagnetic metal and with a intergranule consisting of a nonmagnetic and insulating material.
Among the materials producing the GMR effect pointed out above, the multilayer film represented by the spin valve is featured in its high sensitivity under a low magnetic field. However, for preparing the multilayer film, it is necessary to laminate thin films made of various materials at a high precision, leading to a poor stability and a low manufacturing yield of the multilayer film. Such being the situation, reduction of the manufacturing cost is limited. Under the circumstances, the multilayer film of this kind is used exclusively in a high value-added device such as a magnetic head for a hard disk. It is considered difficult to use the particular multilayer film in a magnetic sensor that is forced to make competition in price with, for example, the AMR sensor or the hole sensor having a low unit price. It should also be noted that diffusion tends to be generated among the layers forming the multilayer film, and the GMR effect tends to disappear, with the result that the multilayer film is poor in its heat resistance.
On the other hand, the nano granular material can be manufactured easily and has a high reproducibility in general. Therefore, the manufacturing cost of the magnetic sensor can be lowered when the nano granular material is used for the manufacture of the magnetic sensor. Particularly, the metal-insulator system nano granular material is advantageous in that (1) if the composition is optimized, the metal-insulator system nano granular material is allowed to exhibit a high MR ratio exceeding 10% under room temperature, (2) since the metal-insulator system nano granular material exhibits an extremely high electrical resistivity, it is possible to miniaturize markedly the magnetic sensor and to save the power consumption of the magnetic sensor, and (3) the metal-insulator system nano granular material can be used even under a high temperature environment unlike the spin valve films comprising an antiferromagnetic film that is poor in its heat resistance. However, the metal-insulator system nano granular material is defective in that the sensitivity to the magnetic field is very low under a low magnetic field.
A measure for overcoming the problems pointed out above is disclosed in Japanese Patent Disclosure (Kokai) No. 11-087804. Specifically, it is disclosed that soft magnetic thin films are arranged on both sides of a giant magnetoresistance effect thin film in order to increase the sensitivity of the giant magnetoresistance effect thin film to the magnetic field. Also disclosed in the patent document quoted above is a method of manufacturing a thin film magnetic sensor, comprising the steps of forming a permalloy thin film (soft magnetic film) in a thickness of 2 μm on a substrate, forming a gap having a width of about 9 μm in the permalloy thin film by using an ion beam etching apparatus, and forming a nano granular GMR film having a composition of Co38.6Y14.0O47.4 in the gap portion.
Japanese Patent Disclosure No. 11-274599 is also directed to a thin film magnetoresistance element in which soft magnetic thin films are arranged on both sides of a giant magnetoresistance thin film. It is taught that, in order to further improve the sensitivity of the magnetoresistance element to the magnetic field, the giant magnetoresistance thin film is made thinner than the soft magnetic thin film.
A soft magnetic material having a large saturation magnetization and a high magnetic permeability has a very high sensitivity to the magnetic field and exhibits a very large magnetization under a relatively weak external magnetic field. Therefore, when an external magnetic field is allowed to act on a thin film magnetic sensor constructed such that a thin film having a high electrical resistivity and producing a giant magnetoresistance effect (GMR film) is arranged in a small gap formed between thin film yokes formed of a soft magnetic material such that the GMR film is electrically connected to the thin film yokes, the thin film yokes are magnetized by a weak external magnetic field, and a magnetic field having an intensity 100 to 10,000 times as high as that of the external magnetic field is exerted on the GMR film. As a result, it is possible to markedly increase the sensitivity of the GMR film to the magnetic field. Incidentally, a metal-insulator system nano granular thin film is known nowadays as the GMR film.
FIG. 1 is a plan view schematically showing the construction of a conventional thin film magnetic sensor 10, and FIG. 2 is a cross sectional view along the line II—II shown in FIG. 1. As shown in FIGS. 1 and 2, the conventional thin film magnetic sensor 10 comprises an insulating substrate 12 made of an insulating and nonmagnetic material, a pair of thin film yokes 14 each formed of a soft magnetic material, the thin film yokes 14 being arranged to face each other with a gap 14a formed therebetween, a GMR film 16 formed within the gap 14a, electrodes 18, 18 formed at the edge portions of the thin film yokes 14, and a protective film 19 for protecting the thin film yokes 14 and the GMR film 16.
The conventional thin film magnetic sensor 10 of the construction described above is formed by the method comprising the steps of forming the pair of the thin film yokes 14 arranged to face each other with the gap 14a (concave groove) interposed therebetween by removing the unnecessary portion of the soft magnetic thin film formed on the surface of the insulating substrate 12, and depositing the GMR film 16 with a mask formed to cover the insulating substrate 12 except the regions in the vicinity of the gap 14a. 
However, the thin film magnetic sensor 10 manufactured by the method described above gives rise to the problem that the electrical characteristics and the magnetic characteristics of the sensor 10 greatly varies. The difficulty is brought about by the situation that, in the conventional manufacturing method described above, for example, the electrical contact between the thin film yokes 14 and the GMR film 16 is rendered insufficient, or the thickness of the GMR film 16 is made nonuniform within the gap 14a, with the result that the manufactured sensor 10 is rendered unstable.
FIG. 3 shows the difficulty accompanying the conventional method of manufacturing the thin film magnetic sensor. To be more specific, if the GMR film 16 is deposited from above the thin film yokes 14 positioned to face each other with the small gap 14a interposed therebetween, the thickness in the side wall portions 16c of the GMR film 16, which are formed on the side walls of the thin film yokes 14 having a large height, is gradually increased in accordance with increase in the thickness of the upper portions 16a of the GMR film 16 deposited on the upper surfaces of the thin film yokes 14, as shown in FIG. 3. As a result, the corner portions at the bottom of the gap 14a are shaded by the side wall portions 16c of the GMR film 16 deposited on the side walls of the thin film yokes 14. It follows that the deposition of the GMR film 16 is inhibited at the corner portions in the bottom portion 16b of the GMR film 16, which is deposited on the bottom surface of the gap 14a. Such being the situation, the bottom portion 16b of the GMR film 16 is rendered triangular or trapezoid in its cross sectional shape so as to cause the contact electrical resistance between the bottom portion 16b of the GMR film 16 and the thin film yokes 14 to greatly vary. Particularly, this undesirable phenomenon is rendered prominent in the high performance type thin film magnetic sensor in which the thin film yokes have a large height and the gap between the paired thin film yokes is small. In the worst case, the electrical resistance is rendered infinitely high so as to give rise to a serious obstacle that must be eliminated for putting the thin film magnetic sensor to the practical use.